1. Field of the invention
The present invention relates a switching regulator, especially a switching regulator which comprises a synchronous rectifier circuit that performs synchronous rectification with a main switching element by using rectifying switching elements as rectifier elements.
2. Description of the Related Art
In this type of switching regulators, relatively large loss is normally caused by the forward voltage of rectifier diodes if such rectifier diodes are used in a rectifier circuit for rectifying a voltage induced in a secondary winding of a transformer. As one representative of prior art for overcoming such drawback, a synchronous rectifier circuit is known in which rectifying switching elements such as Field Effect Transistors (MOS FET), for example, are used as a substitute for the rectifier diodes.
In Japanese un-examined patent publication number 7-115766, for example, is disclosed a flyback type switching regulator, using a certain synchronous rectifier circuit in which when the turn-off of a main switching element is followed by the current flowing through a current transformer via a body diode of a rectifying switching element that comprises MO-SFETs, a voltage is generated according to the current flowing through a resistor connected to a secondary winding of the current transformer, and thus an output of a comparator comparing the voltage with a reference voltage becomes xe2x80x9cHxe2x80x9d(High) level so that the rectifying switching element turns on through a drive circuit; and thereafter, energy stored in the transformer is transferred to an output side and the current flowing through the rectifying switching element becomes zero so that the output of the comparator becomes xe2x80x9cLxe2x80x9d(Low) level and the rectifying switching element is made to turn off. In this case, when the current ceases to flow through the rectifying switching element, a control signal to turn off the rectifying switching element is given, whereby the polarity of voltage across an auxiliary winding is reversed so that a signal to make the main switching element turn on is generated.
Also in Japanese registered patent publication number 2976180 is disclosed a synchronous rectifier circuit wherein a current flowing through a rectifying switching element is detected by a current transformer so that the rectifying switching element may be turned on or off. More specifically, the synchronous rectifier circuit is a circuit where a primary winding of a current transformer is connected in series to a parallel circuit of diode and MOS FET that is a rectifying switching element; a resistor is connected in parallel to the secondary winding of the current transformer; and a buffer amplifier lowering an output impedance of the current transformer is connected to a path between the resistor and the gate of the MOS-FET, whereby when the exciting energy of the transformer that comprises a flyback converter finishes discharging the energy as an electrical energy, the polarity of the output signal of the current transformer is reversed and the rectifying switching element is turned off; and thus, a reverse current is prevented from flowing when an output current becomes so small that a period of the current interruption is produced, so that the regulator efficiency is improved.
As is apparent from the foregoing, the above-mentioned synchronous rectifier circuit is one where the rectifying switching element is not only turned on synchronously with the main switching element but also is forcedly turned off in order to block the reverse current flowing through the rectifying switching element.
In the above-mentioned conventional circuits, however, higher efficiency cannot be attained as the rectifying switching element is turned on after detecting the current flowing through the current transformer and thus the rise time is too slow. Further, whilst the above-mentioned registered patent publication number 2976180 teaches that an output voltage is applied to the gate of the MOS-FET via transistors that construct a part of a buffer amplifier when a current flows through the current transformer, it is not possible to drive the rectifying switching element by lower output voltages such as DC 2 V and 3.3 V when such lower output voltages are strongly requested recently.
Also, in the case of a switching regulator in which component regulators including such synchronous rectifier circuit perform a parallel running, the unbalance in the output currents between the component regulators has a serious effect on the regulator as a whole. That is: as a MOS-FET serving as a rectifying switching element has a characteristic that a current is permitted to flow bidirectionally between a drain and a source (bidirectional conductibility), the other circuitry elements in the regulator are likely to be damaged by the reverse current flowing through the rectifying switching element from other component regulators while the rectifying switching element is on.
With regard to the problem, in the Japanese un-examined patent publication number 7-75336 is proposed a regulator wherein even though a higher voltage than an output voltage of a certain regulator itself is applied from output terminals of other regulators, a main switching element is switched by on-pulse of minimum width in each cycle, thus preventing the continuous flow of direct current over two cycles in the rectifying switching element. In this case, however, there must be provided an additional function to keep the main switching element xe2x80x9conxe2x80x9d by on-pulse of a minimum width under any condition, thus leading to likelihood that the circuit topology becomes too complicated.
To eliminate the above-mentioned problem, it is, therefore, an object of the present invention to provide a switching power supply that can operate by lower voltage, while preventing a reverse current from flowing.
It is another object of the present invention to provide a switching power supply with a simple circuit topology that can prevent harmful effects caused by the reverse currents flowing from other component regulators in a parallel running.
According to a first aspect of the present invention, a switching power supply has a rectifying switching element used as a rectifier element that is connected to a secondary winding of a transformer, and includes a synchronous rectifier circuit. The switching power supply also includes a current transformer for detecting a current flowing through the rectifying switching element, with an auxiliary winding wound in the transformer. A drive circuit is included for feeding a voltage generated in the auxiliary winding to the rectifying switching element as an xe2x80x9conxe2x80x9d signal. A turn-off circuit is also included for turning off the rectifying switching element before the current flowing through the rectifying switching element reverses, following a detection signal generated in a secondary winding of the current transformer.
When a current starts flowing through the rectifying switching element, the rectifying switching element is built up abruptly by the voltage that is induced. The current is not induced in the current transformer due to it having a high impedance but in the auxiliary winding which has a low impedance, so that the rectifying switching element turns on immediately when the current approaches close to a maximum value. Consequently, the resistance of the rectifying switching element in an xe2x80x9conxe2x80x9d state can be reduced as compared with that in the conventional drive circuit by a current transformer so that not only the rectifying switching element but also the power supply itself can have improved efficiency.
Further, as the voltage induced in the auxiliary winding is fed directly to the rectifying switching element as a drive signal, voltage sufficient to turn on the rectifying switching element can be fed merely by suitably changing the number of turns of the auxiliary winding without being influenced by the output voltage so that the needs for a lower output voltage in power supplies can be met. Besides, the rectifying switching element turns off before the current flowing therethrough is reversed, thus ensuring the prevention of the reverse current flowing.
According to a second aspect of the present invention, the switching power supply includes a unidirectionally conductible element which interrupts an output of a signal from the turn-off circuit to the drive circuit when the rectifying switching element is on, while the unidirectionally conductible element conducts to turn off the switching element when a detection signal induced in the secondary winding of said current transformer drops.
Thus, the voltage induced in the auxiliary winding is fed to the switching element as an xe2x80x9conxe2x80x9d signal without influencing the drive circuit of the switching element. As the rectifying switching element turns off when the detection signal generated in the secondary winding of the current transformer is lowered, further stabilization of the on/off control can be simply attained by simply installing an unidirectionally conductible element.
According to a third aspect of the present invention, the switching power supply includes an emitter-follower which is provided in the drive circuit of the rectifying switching element and which is connected to a path between the auxiliary winding and the rectifying switching element.
In this case, as the emitter-follower circuit acts as a circuit matching the impedance of the auxiliary winding to that of the rectifying switching element, the rectifying switching element is built up more rapidly than that in conventional switching power supply so that the efficiency is further improved
According to a fourth aspect of the present invention, the switching power supply includes a thyristor circuit which is provided in the drive circuit of the rectifying switching element and which is connected to a path between the auxiliary winding and the rectifying switching element.
In this case, once a voltage induced in the auxiliary winding is fed to the gate of the rectifying switching element as a driving voltage, the gate voltage of the rectifying switching element is kept as it is so that it takes an approximately rectangular waveform. Accordingly, the ON-state resistance of the rectifying switching element is kept smaller even after it is turned on so that the efficiency can be improved further.
According to a fifth aspect of the present invention, the switching power supply includes a differential circuit which is provided in the turn-off circuit of the rectifying switching element and which is connected across the secondary winding of the current transformer.
In this case, positive and negative trigger signals are generated from the differential circuit, corresponding to the rise and fall of current generated in the secondary winding of the current transformer. Accordingly, the rectifying switching element is turned off in the case that a negative trigger signal is generated so that on/off operation in the rectifying switching element can be stabilized further.
According to a sixth aspect of the invention, the switching power includes a switching means which is provided in the drive circuit and which turns on when the level of detection signal generated in the secondary winding of the current transformer drops, and then discharges electric charge, stored in the gate of the rectifying switching element, to an output side.
In this case, the electric charge, stored in the gate of the rectifying switching element, can be discharged to an output side via switching means when the rectifying switching elements turns off.
According to a seventh aspect of the invention, the switching power supply includes a speed-up circuit is provided in the turn-off circuit of the rectifying element and which is composed of a parallel circuit of a resistor and a capacitor connected to a path between the secondary winding of the current transformer and the switching means.
In this case, the electric charge in the gate of the rectifying switching element can be rapidly discharged to an output side by speeding up the switching action in the switching means owing to the speed-up circuit.
According to an eighth aspect of the invention, the switching power supply includes a plurality of switching component power supplies performing a parallel running, each component power supply being provided with the synchronous rectifier circuit.
Accordingly, when a rectifying switching element in one component power supply, for examples, turns on in a parallel running where output current is fed to a common load from a plurality of component power supplies, the reverse currents from other component power supplies might be allowed to flow into the component power supply via the rectifying switching element of the component power supply, and yet, such reverse currents are detected by the turn-off circuit of the rectifying switching element. Thus, the reverse currents from other component power supplies can be prevented from flowing into the component power supply so that harmful influence on other circuit elements of the component power supply can be avoided, with the foregoing simple structure of the current transformer and the turn-off circuit.
According to ninth aspect of the invention, the turn-off circuit of the switching power supply is so composed that the rectifying switching element may be turned off when a forward current flowing through the rectifying switching element drops to a predetermined level or below.
Accordingly, not only reverse currents from other component power supplies can be prevented from flowing into the component power supply but also the off-timing of the rectifying switching element is determined by the common turn-off circuit thereof. Thus, the circuit topology does not need to be complicated.
According to a tenth aspect of the invention, a switching power supply has a main switching element, a choke coil for charging or discharging energy, corresponding to on or off of the main switching element and a rectifying switching element which turns on when the choke coil discharges energy. The switching power supply includes a synchronous rectifier circuit which is composed of a drive winding wound in the choke coil, a current transformer for detecting a current flowing through the rectifying switching element, a drive circuit which feeds a voltage induced in the drive winding to the rectifying switching element as an xe2x80x9conxe2x80x9d signal, and a turn-off circuit which turns off the rectifying switching element by a detection signal induced in the secondary winding of the current transformer.
Thus, when the main switching element turns from xe2x80x9conxe2x80x9d to xe2x80x9coffxe2x80x9d, a voltage is induced in the drive winding by the energy stored in the choke coil until that time point. Then, this voltage is fed from the drive circuit to the rectifying switching element so that the rectifying switching element turns on. On the other hand, when an inertial current flowing through the rectifying switching element, along with the discharge of energy from the choke coil, is detected by the current transformer, and the detection signal induced in the secondary winding of the current transformer drops down to a certain level or below, then the rectifying switching element turns off. By clearly determining the on-off timing of the rectifying switching element this way, a useless power loss due to a reverse current from a light load, for example, allowing energy to be stored in the choke coil and then the energy returning back to a power source, can be avoided.
In addition, by simply winding the drive winding in an existing choke coil, the voltage induced in the drive winding can be directly utilized as the drive signal for driving the rectifying switching element. Also, by simply changing the number of turns of the drive winding in a suitable manner, a voltage sufficient to turn on the rectifying switching element can be fed, so that needs for lower output voltage in power supply can be met through such slight modifications in design.